Hydroxyapatite (Ca10(PO4)6(OH)2), obtainable by sintering of a calcium phosphate powder, is a main component of biological hard tissues such as bones and teeth and has biocompatibility. With this in mind, the use of hydroxyapatite as a repair material for a missing part or a cavity in a biological hard tissue has been reported. A material including hydroxyapatite as thus described is indeed excellent in biocompatibility but insufficient in formability, and may therefore be difficult to apply to a repair area of complicated shape.
Cement-type calcium phosphate compositions, i.e., calcium phosphate compositions having curability, are known to gradually convert to bioabsorbable hydroxyapatite in a living body or an oral cavity and be able to unite with a biological hard tissue while retaining its shape. Such curable calcium phosphate compositions not only have excellent biocompatibility but also have formability, and are therefore considered easy to apply to an area of complicated shape.
Various types of curable calcium phosphate compositions have been developed thus far. For example, Patent Literature 1 describes a rapid-curing calcium phosphate cement inducing at least one of a calcium hydrogen phosphate anhydrous powder and a calcium hydrogen phosphate dihydrate powder which have a specific surface area of 1 to 50 m2/g. Even when kneading is performed with only the addition of water to a liquid material but without the addition of any acid, this cement rapidly cures, and a cured product of calcium phosphate having a high compressive strength can be obtained. However, the rapid-curing calcium phosphate cement described in Patent Literature 1 has a problem in that the strength of the cured product decreases over time in a wet environment.
Patent Literature 2 describes a calcium phosphate powdery composition composed of tetracalcium phosphate particles and calcium hydrogen phosphate particles, the calcium phosphate powdery composition containing 0.001 to 1 parts by weight of water per 100 parts by weight of the total of the tetracalcium phosphate particles and the calcium hydrogen phosphate particles. The curing time of this composition is in an appropriate range, and its workability is good. Additionally, with this composition, a cured product of calcium phosphate having high mechanical strength is obtained. However, the calcium phosphate powdery composition described in Patent Literature 2 has room for improvement in that the strength of the cured product decreases over time in a wet environment.
Patent Literature 3 describes a calcium phosphate cement which self-cures to hydroxyapatite at ambient temperatures, the calcium phosphate cement including an additional source of calcium and a calcium phosphate salt having a calcium-to-phosphorous molar ratio of less than 5/3 which is other than tetracalcium phosphate. However, the calcium phosphate cement described in Patent Literature 3 has a problem in that the strength of the cured product decreases over time in a wet environment.
Patent Literature 4 describes a dentinal tubule sealing material containing poorly-soluble calcium phosphate particles, a phosphorus-free calcium compound, and water. According to this literature, it has been confirmed that when the sealing material obtained as a paste is rubbed on dentin with a microbrush for 30 seconds, dentinal tubules can be sealed with the sealing material, and a high dentin penetration inhibition ratio can be achieved. However, the dentinal tubule sealing material described in Patent Literature 4 has room for improvement in terms of the durability of the resulting sealing product.
Patent Literature 5 proposes a method including: mixing a powder containing tetracalcium phosphate particles and an alkali metal phosphate with a liquid containing water as a main component; and applying the obtained aqueous paste to dentin surface. This method enables sealing of a large proportion of dentinal tubules, and can therefore be expected to effectively inhibit pain. However, the method described in Patent Literature 5 has room for improvement in terms of the durability of the resulting sealing product.